Osteogenesis Imperfecta (OI) is a heterogeneous genetic disease of humans that results in bone fragility. There is increasing evidence that structural and regulatory mutations of type I collagen are responsible for the clinical features of the disease. These abnormalities are expressed in culture fibroblasts from affected individuals and can be used to probe the structure and function of the type I collagen gene. In this grant, five OI strains with specific abnormalities of type I collagen synthesis will be studied in detail. The cells express: low type I synthesis without a structural abnormality; low type I synthesis with a structural abnormality of the alpha 1 chain; a deficiency in alpha 2 chain synthesis; a point mutation in the helical region of the alpha 2 chain; low alpha 2 chain synthesis without a structural abnormality. The mRNA from each cell strain will be ascessed for the content (saturation hybridization) and function (cell-free translation) of type I procollagen mRNA. In cells with low alpha 1 or alpha 2 procollagen mRNA content, the rate of procollagen mRNA formation and degradation plus the gene copy number will be determined. In cells where abnormal structure or impaired function is present, the structure of the mRNA will be studied using S-1 protection experiments. Implicit to the success of this project is the development or acquisition of appropriate DNA probes. We have obtained DNA probes from investigators to initiate some of the studies. However, our initial activitiy will continue efforts to develop human cDNA and genomic probes to type I collagen sequences. As our understanding of the abnormalities of the type I collagen gene structure and function present in these OI cell strains improve, it will be possible to develop specific biochemical markers for the disease that will permit improved clinical classification and informed genetic counseling.